Service manual

Page 15
user (the mic’s /PTT) or when the Keyer is controlling it. An output port bit, PTTO, or the
Keyer, via PTT_Keyer, are used to generate LTxEn, which is fed to the 10W transmitter as well
as the 100W amplifier (Sheet 7).
The Keyer microprocessor also controls when the receiver is enabled, through the signal
HRcvEn. The two control signals, LTxEn and HRcvEn, are sequenced so that the receiver is
muted before the transmitter turns on, and so that the transmitter has time for the RF to make it
all the way through the various amplifier stages (and so that RF bleedthrough into the receiver
stages also have time to propagate) before the receiver is un-muted and its AGC controls re-
turned to normal.
Sheet 4 also shows the 5V to 3.3V level converter (U10) that drives address and data lines on
the DDS chips.
A-to-D (A/D) Converters
Going back to Sheet 2, the Mega644P has eight A/D converter inputs. There is actually only
one 10-bit A/D inside the chip, and it uses a multiplexer to look at one analog input at a time.
The signals fed into the first seven inputs are the FM squelch, AF Gain, RF Gain, Mic Gain,
Headphone Volume, RF Power and RF Processor pots that are located on the Front Panel board
and fed into the Controller on J9 (Sheet 6). The eighth analog input comes from U19 on Sheet
7. This CMOS analog multiplexer selects one of eight additional voltages: Forward and Re-
flected Power from the DCD/Tuner board; Supply voltage, ALC/compression and Driver Cur-
rent from the 10W transmitter board; Final Current from the 100W PA board, S-Meter value
from the Receiver board, and the value of the IF Filter switch on the Front Panel. An interrupt
routine in the microprocessor selects and samples one of these 15 voltages every time an A/D
conversion completes (about every 64 microseconds).
Parallel Data Bus
Also on Sheet 2, Port C of the processor is used as an 8-bit parallel data bus for all boards. This
bus is buffered by U15 to become the “XBus”. Its output is enabled only when data on one of
the boards needs to be changed, otherwise pull-up resistors on each board terminate the bus and
keep it quiet to avoid causing unnecessary digital interference to the transmitter or receiver.
U20 is an address decoder. AS0, AS1 and AS2 (U34, Sheet 6) select one of eight data strobe
pulses for the DDS chips and read/write pulses to the Keyer microprocessor.
I/O Strobes, inter-processor communications and address decoding
Port B on the microprocessor is used for I/O control. IOStrobe1 and /IOStrobe2 serve as clock
pulses to output data from Port C to the various I/O chips on the Controller board. (The “/” in
front means that the signal is low true, meaning that it is normally high and pulses low when
active.) DispRDY and /DispCk are signals from and to the Vacuum Fluorescent Display board
that are fed to the Controller board from J17 on Sheet 7. KeyerRDY is a signal from the Keyer
microprocessor (U30, pin 42, Sheet 3) that tells the main processor that data is either ready on